Bimetallic connector for completing path between cathode and heat sink for temperature control



United States Patent Inventor Nicholas P. Pappadis Chicago, Ill.

Appl. No. 756,461

Filed Aug. 30, 1968 Patented Dec. 22, 1970 Assignee Zenith Radio Corporation Chicago, 111. a corporation of Delaware BIMETALLIC CONNECTOR FOR COMPLETING PATH BETWEEN CATHODE AND HEAT SINK FOR 13,82, 82BFC; 315/72 Primary Examiner-Robert Segal Attorney-Francis W. Crotty ABSTRACT: A temperature stabilized cathode assembly for a cathode-ray tube is conventional except for the addition of a bimetallic element for establishing a path of low thermal resistance between the cathode and a heat sink. One end of the bimetallic element is fixed and the other is in close proximity to but out of physical contact with the cathode. As the cathode comes to temperature and this element expands, a desired low resistance thermal connection is established to increase the heat dissipating properties of the cathode. If the cathode temperature goes beyond a desired value, further expansion of the bimetallic element breaks the connection to the heat sink, restoring the normal temperature heater voltage characteristic of the cathode.

I i IE "9 r-i 18 l6- F a I r T E l0 PATENTED 05022190 FIG, 5

5.1 o FILAMENT VOLTAGE Inventor Nucholas P. Puppodls m n I p 1 EIMETALILIC CONNECTOR FOR COMPLETING PATH BETWEEN (IATIIODE AND I-IEAT SINK FOR TEMPERATURE CONTROL BACKGROUND OF THE INVENTION The present invention is directed broadly to temperature stabilization of an electronic device electrode such as the cathode assembly of an electron discharge device. The need for heat stabilization is particularly acute for the cathode assembly of a cathode-ray tube and, for convenience, the invention will be described in that environment.

The cathode element of the ordinary cathode-ray tube is of the indirectly heated type, featuring a cathode cylinder which bears an oxide coating of a suitable composition at one end and is open at the other end to facilitate the insertion of a heater which, when energized, raises the temperature of the cathode cylinder and the emitting material to an appropriate operating temperature at which electrons are given off. There is an optimum operating temperature for such a cathode and it is desirable that the cathode have a temperature-filament or heater-voltage characteristic with a fairly steep slope to the end that the desired operating temperature is attained quickly after the heater has been energized. It is-undesirable to exceed the optimum operating temperature because operation at higher temperatures tends to shorten tube life by dissipation of the electron emitting material of the cathode. It is also known that operation below the optimum temperature is undesirable because under such conditions the cathode is subject to gas poisoning and certain undesirable atmospheric effects within the tube. Therefore, it is evident that an attractive cathode is one which quickly attains optimum operating temperature and stabilizes at that temperature so as to be relatively-insensitive to fluctuations of heater voltage which, in turn, reflect changes in the operating or line voltage energizing the equipment which includes the cathode structure.

Efforts to attain such a characteristic usually include some mechanism by which the heat dissipating properties of the cathode tend to increase once the heater has been energized and the temperature of the cathode starts to increase. For example, it may be arranged that the contact area between the cathode and a heat sink be increased as the temperature of the cathode increases. A number of mechanical devices suggest themselves to accomplish that result such as an expandable connection between the cathode and the heat sink so that the area of contactenlarges with increased temperature.

Such prior efforts to stabilize a cathode assembly are subject to two shortcomings. They tend to exert their influence as soon as the cathode starts to heat and, having once become effective, they remain so at whatever temperature the cathode attains. It is much preferred that the heat stabilizer not become effective until the cathode is brought pretty close to its desired operating temperature since in this way operating conditions are most quickly established. It is further highly desirable that the heat stabilizer become ineffective when the heater voltage exceeds the range of fluctuations that is normally encountered in the operation of the equipment including such a cathode. This is desirable because in the processing of the cathode it is customary and necessary to apply an abnormally high heater voltage to accomplish processing of the emitting material. If the heat stabilizer were effective during this step of the cathode processing, it would be exceedingly difficult to process the cathode without at the same time destroying the heater.

Accordingly, it is an object of the invention to provide an improved temperature stabilized cathode assembly for an electron discharge device, such as a cathode-ray tube.

It is a specific objective of the invention to provide an improved cathode assembly which is temperature stabilized but essentially only over the range of operating voltages expected to be encountered in service.

SUMMARY OF THE INVENTION In accordance with the invention, a temperature stabilized cathode assembly for an electron discharge device, such as a cathode-ray tube, comprises a cathode having a heater and further having on one surface a coating of material capable of emitting electrons when the cathode is brought to a predetermined operating temperature The cathode has a temperatureheater voltage characteristic with a positive slope. The assembly further includes means, responsive to the heating of the cathode to a temperature at least approximately equal to the desired operating temperature, for increasing the heat dissipating properties of the cathode to reduce the slope of its temperature-voltage characteristic.

Structurally, heat stabilization is obtained through the use of a bimetallic element that may connect the cathode to a heat sink. This connection is normally brolten but, as the cathode attains operating temperature, the bimetallic element expands to complete the connection and effect the desired heat stabilization.

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several FIGS. of which like reference numerals identify like elements, and in which:

FIG. 1 is a fragmentary view partially in cross section of an electron gun for a cathode-ray tube featuring temperature stabilization in accordance with the subject invention;

FIG. 2 includes curves representing the temperature-heater voltage characteristic of the cathode assembly of FIG. l; and

FIGS. 3, 4 and 5 represent other forms of the temperature stabilized cathode assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the invention is to be described in connection with temperature stabilization as the cathode assembly of a cathode-ray tube, it may be completely understood with reference to the electron gun of such a tube and, therefore, FIG. 1 is confined to a showing of only that portion of a picture tube. Its cathode comprises a metallic cylinder ill) which is closed at one end by means of a cap 11 which has on the exposed end surface thereof a coating 11a of a material capable of emitting electrons when the cathode is brought to a predetermined operating temperature. It is desirable that cylinder 10 have low thermal conductivity and it may, therefore, be formed of a nickel-iron alloy. The emitter coating llla is one of the commercially available oxide emitters in use today. Generally, the coating is applied in a carbonate form and is converted to an oxide during tube processing; the final coating is a mixture of barium oxide, strontium oxide and calcium oxidehOne end of cylinder i0 'is open to receive the customary heater or filament 12 having terminals T3 to which an energizing voltage is applied from a source (not shown). The cathode is surrounded by a coaxially supported cylindrical shield 14 which has relatively few, perhaps three, small 'area contacts with cathode cylinder 10 and their connection,

as illustrated, occurs near the open end of the cathode. At its opposite end shield M has a flange 16 to facilitate its connection with metallic tabs l7 by means of which the described cathode structure may be supported from the usual insulating pillars 13.

Following the cathode structure, the gun under consideration includes a first or control grid 2%, a second grid 2i, and a grid 3 or focus electrode 22. While not: shown, electrode 22 is followed by a fourth electrode referred to as an anode. As thus far described, the electron gun of FIG. l is entirely conventional both as to its structure and mode of operation and, consequently, no further discussion thereof is required. The

remainder of this description will be directed to the teaching of the invention for effecting temperature stabilization of the cathode assembly.

It is convenient at the outset to refer to FIG. 2 in which curve A represents the temperatureheater or filament-voltage characteristic of the cathode which has a uniform and positive slope. This slope is rather steep and is typical of commercially available cathode structures so that an optimum operating temperature T is quickly attained once the filament 12 is energized by the application of heater voltage to its terminals 13. If it be assumed that the heater voltage is subject to fluctuation over a particular range, shown in FIG. 2 as extending from 5.7 to 7.0 volts, it is apparent that the steep slope of curve A causes the heater to be undesirably sensitive to such voltage fluctuations. In a construction embodying the invention, however, the characteristic, instead of being linear, has a first point of inflection at the low end of the normal range of heater voltages and, ideally, the characteristic becomes flat at that point and stays flat over the entire range as indicated by broken-line curve B. Also as shown'by that curve, a second point of inflection is provided at the high end of the range of heater voltages to permit processing of the cathode during the fabrication of the tube. A curve of this type may be realized by use of the invention in accordance with which the cathode assembly includes 'means, responsive to the heating of the cathode to a temperature approximately equal to the desired operating temperature T for increasing the heat dissipating properties of the cathode to reduce the slope of its temperature-heater voltage characteristic. As stated, ideally this slope is reduced to zero.

In the embodiment of FIG. 1, this means comprises a curved bimetallic strip 25 having an element 25a of stainless steel which has a high'coefticient of expansion and an element 25!; of invar which, by comparison, hasa low coefficient of expansion. One end of the bimetallic strip is fixed to tab 17 through which the cathode assembly is supported from pillars l8 and the free end of the strip is in close proximity too but normally out of physical contact with the cathode assembly, assuming the term normally to describe the deenergized state of the cathode. Because of the different coefficients of expansion of elements 25a and 25b an increase in the temperature of strip 25 causes expansion and displacement of the free end thereof to positions shown in broken construction line. It will be observed that in one such position the free end of strip 25 contacts the cathode assembly; specifically, it contacts the cathode shield 14 which may have a domed elemental area 14a to permit contact with the free end of strip 25 in one position in its range of displacement while yet making it possible for further expansion of the strip to displace the free end, shifting it clear of the entire cathode structure.

In operation, at the time the cathode is initially energized, strip 25 is in its full-line positionout of physical contact with the cathode assembly. As the cathode comes to temperature, the ambient temperature increases and expands the bimetallic strip, causing its free end to contact cathode portion Ma at a time when the cathode shall have attained its desired operating temperature T This, in effect, connects the cathode to a heat sink comprised of mounting elements 17 and if the parameters of the structure are properly selected, the characteristic of curve A is modified to that of curve B with a flat rather than a sloping part over the range of expected heater voltage fluctuation. In particular, it is desirable that the characteristic not droop over the expected range of heater voltage.

The parameters that are determinative of the characteristic include such things as the width and cross section of the materials constituting strip 25. They further include the contact area of the strip with the cathode and the point at which the contact is made in terms of the temperature gradient along the cathode cylinder. The heat sink must have adequate capacity to increase the heat dissipation of the cathode the proper amount. Of course, it is necessary that strip 25 and the heat sink with which it connects have low thermal resistivity in order to efficiently conduct heat from the cathode when it is desirable to hold a constant operating temperature and it is also desirable that the heat sink have efficient heat radiating properties.

Obviously, if voltage in excess of the normal range should be applied to the heater, as of course takes place in processing of the cathode, the temperature of the cathode will tend quickly to exceed its optimum value T resulting in still further expansion of strip 25. Consequently, the free end of the strip is displaced out of contact with portion 14a of the cathode heat shield which breaks the connection from the cathode assembly to the heat sink. In other words, the bimetallic strip causes the cathode to have augmented heat dissipating properties when the temperature has achieved its optimum value T and so long as the heater voltage is within its expected range of fluctuations. In values outside of that range, however, at both the upper and lower ends, strip 25 disconnects the cathode assembly from the heater sink so that the cathode exhibits only its normal heat dissipation.

The embodiment of FIG. 3 differs from that of FIG. 1 only in the configuration of the bimetallic component. In this case it is essentially a disc affixed at the open end of cathode cylinder 10. The disc is dimensioned to deflect into engagement with the mounting straps 17 when the cathode is at its desired and intended operating temperature. The mounting straps are shaped so that further deflection of disc 25, experienced at cathode temperatures in excess of T causes the disc to pass beyond and out of contact with the mounting straps.

Very similar results are obtained with the modification of FIG. 4 in which a heat conductive element 26 is secured along the length of bimetallic strip 25 in alignment with an aperture 14b of cathode shield 14. Additionally, a stop 27 is affixed to shield 14 for engagement with the free end of strip 25. In this modification, initial expansion of strip 25 as the cathode achieves its desired temperature T displaces element 26 into contact with cathode cylinder 10. This establishes a connection with the heat sink and introduces the flat section of the temperature-heater voltage characteristic that is desired. The second inflection point of the characteristic comes about because as the cathode exceeds its optimum temperature appreciably strip 25 further expands and its free end is brought to bear against stop 27. This results in a bowing of strip 25 as a consequence of which conductive element 26 is withdrawn from cathode cylinder 10 breaking its connection with the heat sink.

In the embodiment of FIG. 5 a two section bimetallic component is employed. The two sections are designated respectively with prime and double prime reference characters. It is apparent that the sections deflect in opposite directions under the influence of heat since the high expansion portion 25a of one section is away from the cathode, whereas the high expansion portion 25a" of the other section is adjacent the cathode. In the operation of this structure, when the cathode achieves its operating temperature T section 25' deflects toward the cathode and causes the other section to contact the cathode structure, connecting it to the heat sink. At higher temperatures the other section 25" deflects away from the cathode and breaks the connection of the cathode structure to the heat sink. Thus, this modification also effects the desirable two points of inflection in the characteristic.

While particular embodiments of the invention have been shown and described it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

lclaim:

1. A temperature stabilized cathode assembly for an electron discharge device, such as a cathode-ray tube, comprising:

a cathode having a heater and further having on one surface a coating of material capable of emitting electrons when said cathode is brought to a predetermined operating temperature, said cathode having a temperature-heater voltage characteristic with a positive slope;

heat sink means for connection with said cathode to reduce the slope of said temperature voltage characteristic; and

a heat responsive expandable bimetallic connector fixed at one end to one of said heat sink means and cathode and displaceable, in response to the heating of said cathode to said predetermined temperature, from a first to a second position in which a heat conducting path is established from said cathode through said connector to said heat sink means and which is further displaceable, in response to the heating of said cathode to a temperature exceeding said predetermined temperature by a preselected amount, to a third position in which said path is interrupted.

2. A cathode assembly in accordance with claim 1 in which said connector is a curved bimetallic strip fixed at one end to said heat sink means and having afree end displaceable over a path in which said free end contacts said cathode when said connector is in said second position and then passes beyond and out of contact with said cathode as said connector is disiii placed to said third position. I

3. A cathode assembly in accordance with claim 1 in which said connector comprises a bimetallic strip fixed at one end to said heat sink means but free at the other end, a heat conductive element secured intermediate the ends of said strip and movable into engagement with said cathode as said strip is dissaid cathode and said sections having such relative lengths that said free end is displaced to establish a connection between said cathode and said heat sink means as said cathode attains said predetermined temperature and is displaced to break said connection as said cathode'temperature exceeds said predetermined temperature. 

